One of Gordon Brown's closest aides has warned that Labour on its current course will lose the next election and be out of power for 15 years, since voters have lost trust in the party and will no longer listen to its message.

The warning by Michael Wills, a former Home Office minister and the Labour MP for Swindon North, is the most public disclosure yet of the deep concern in the chancellor's circle that Labour may lose the election unless there is a radical renewal of the party.

Mr Wills said: "The trouble with the current approach is that we will go out of power and we will go out of power for 15 years." He warned that the party could not afford such a fate, suggesting the Tories would "salami-slice massively redistributive tax credits out of existence".

Mr Brown played a role in encouraging Mr Wills to gain his seat and has subsequently seen him as an important adviser on British identity and the public mood.

At a fringe meeting during a weekend conference organised by Compass, a leftwing pressure group, Mr Wills claimed that at the last election "every single Labour MP on the doorstep reported profound disillusionment and disengagement. We scraped through. If 14,000 Labour voters had voted Tory in May 2005, we would have been in a hung parliament. That is all it took - that is how narrow it was".

He continued: " We have got good messages and we are delivering on public services so why is it they don't listen any more? It is because they don't trust us. Iraq is an important part of that. The presidential style of the prime minister - which brought us great dividends in the early years and now we are seeing the mirror image of that - is also part of it."

He added: "Unless we can get people to start listening to us, unless they are prepared to hear the messages we are putting across, we are going to lose next time. There is no question about it."

He recounted how the American pollster Frank Lunz recently conducted focus groups for BBC Newsnight. He recalled footage of Mr Blair saying "the usual things about the health service and public service reform", which "got uniformly negative readings all the way through. So Frank Lunz asked them 'why are you responding so terribly like this, don't you like what he is saying ? What they said is, 'it is him, it is you' - this is the experience actually most Labour politicians have on the doorstep with most Labour ministers, not just with the prime minister".

In an indication of what he believes the chancellor will need to do, he added that the electorate "have got to believe Labour politicians are on their side and 'understand concerns of people like us' ... They don't believe that at the moment".

The consequences of victory for David Cameron, he said, would be a serious assault on Labour's progressive tax reforms: "The trouble with tax credits - which are very welcome and have a massively redistributive effect - is that they are very complicated and it will be dead easy for the Tories to salami-slice them out of existence over 10 years."

He described the Iraq war as a seminal moment, costing him 3,500 votes in his own constituency. Some MPs are urging Mr Brown to mark a break over Iraq in the event of his becoming prime minister by admitting explicitly that mistakes were made.